Field emission element having carbon nanotube and manufacturing method thereof

ABSTRACT

A method for manufacturing a field emission element, the method includes providing one supporting member and wrapping a carbon nanotube (CNT) film around an outer surface of the supporting member at least once. The CNT film includes a plurality of bundles of carbon nanotubes connected in series.

RELATED APPLICATION

This application is a divisional application of patent application Ser.No. 11/766,996 filed on Jun. 22, 2007 from which it claims the benefitof priority under 35 U.S.C. 120. The patent application Ser. No.11/766,996 in turn claims the benefit of priority under 35 USC 119 fromChinese Patent Application 200610061305.9, filed on Jun. 23, 2006.

BACKGROUND

1. Technical Field

The invention relates to field emission elements and manufacturingmethods thereof and, particularly, to a field emission element employingcarbon nanotubes and a manufacturing method thereof.

2. Description of the Related Art

Carbon nanotubes (CNTs) produced by means of arc discharge betweengraphite rods were first discovered and reported in an article by SumioIijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature,Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes are electricallyconductive along their length, are chemically stable, and can each havea very small diameter (much less than 100 nanometers) and a large aspectratio (length/diameter). Due to these and other properties, it has beensuggested that carbon nanotubes can play an important role in fieldssuch as microscopic electronics, field emission devices, thermalinterface materials, etc.

Generally, a CNT field emission element includes a conductive cathodeelectrode and a carbon nanotube formed on the cathode electrode. Thecarbon nanotube acts as an emitter of the field emission element. Themethods adopted for forming the carbon nanotube on the conductivecathode electrode mainly include mechanical methods and in-situsynthesis methods. One mechanical method is performed by using an atomicforce microscope (AFM) to place a synthesized carbon nanotube on aconductive cathode electrode and to then fix the carbon nanotube on theconductive cathode electrode, via a conductive paste or adhesive. Themechanical method is relatively easy/straightforward. However, theprecision and efficiency thereof are relatively low. Furthermore, theelectrical connection between the conductive base and the carbonnanotube tends to be poor because of the limitations of the conductiveadhesives/pastes used therebetween. Thus, the field emissioncharacteristics of the carbon nanotube are generally unsatisfactory.

One in-situ synthesis method is performed by coating metal catalysts ona conductive cathode electrode and directly synthesizing a carbonnanotube on the conductive cathode electrode by means of chemical vapordeposition (CVD). The in-situ synthesis method is relatively easy.Furthermore, the electrical connection between the conductive base andthe carbon nanotube is typically good because of the direct engagementtherebetween. However, the mechanical bonding between the carbonnanotube and the conductive base often is relatively weak and thusunreliable. Thus, in use, such a carbon nanotube is apt, after a periodof time, to break away (partially or even completely) from theconductive cathode electrode, due to the mechanical stress associatedwith the electric field force. Such breakage/fracture would damage thefield emission electron source and/or decrease its performance.Furthermore, in the in-situ synthesis method, controlling of the growthdirection of the carbon nanotube is difficult to achieve during thesynthesis process. Thus, the production efficiency thereof can berelatively low, and the controllability thereof is often less thandesired. Still furthermore, the in-situ synthesis method has arelatively high cost.

What is needed, therefore, is a field emission element that promotes agood mechanical and electrical connection between the carbon nanotubeand the conductive cathode electrode and that, thus, tends to havesatisfactory field emission characteristics.

What is also needed is a method for manufacturing the above-describedfield emission electron source, the method having a relatively low cost,relatively high production efficiency, and an improved controllability.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referencesto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments.

FIG. 1 is an isometric view of a field emission element, in accordancewith an exemplary embodiment of the present device, the field emissionelement incorporating a CNT field emission layer.

FIG. 2 is a longitudinal sectional view of the field emission element ofFIG. 1, along line II-II.

FIG. 3 is a flow chart showing a method for manufacturing the fieldemission element of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one preferred embodiment of the invention, in oneform, and such exemplifications are not to be construed as limiting thescope of the invention in any manner.

DETAILED DESCRIPTION

Reference will now be made to the drawings to describe embodiments ofthe present field emission element and the related manufacturing method,in detail.

FIG. 1 is an isometric view of a field emission element 10, inaccordance with an exemplary embodiment of the present device, and FIG.2 is a longitudinal sectional view of the upper portion of the fieldemission element 10 of FIG. 1. As shown in FIGS. 1 and 2, the fieldemission element 10 includes one supporting wire 12 and a field emissionlayer 14 coated or otherwise formed on/to an outer surface of thesupporting wire 12. The field emission layer 14 is used for emittingelectrons, and the supporting wire 12 is used for supporting andprotecting the field emission layer 14 and for providing ahigh-conductivity path. It is to be understood that, within the scope ofpresent field emission element, a plurality of the field emission layers14 could, in fact, be formed on a given supporting wire 12 (for example,to achieve a material property gradient or to improve durability).

The supporting wire 12 is advantageously made of a material selectedfrom a group consisting of copper, silver, gold, nickel, molybdenum, orother chemically-durable metal materials. Alternatively, the supportingwire 12 can also be made of glass and/or ceramic. The supporting wire 12is, usefully, thread-shaped, and a diameter thereof is, advantageously,in the approximate range from tens of microns to a few millimeters. Itis, however, to be understood that an even smaller diameter (e.g.,nano-scale) for the supporting wire 12 could potentially be employed,which could allow for a greater emitter density (i.e., emitters pergiven area) to be created on a given field emitter device (not shown),while still providing an improved level of emitter support.

The field emission layer 14 may, for example, be a CNT-polymer compositeor a CNT-glass composite, and a thickness thereof is usually in theapproximate range from 1 micrometer to 1000 micrometers. That said, itis possible for the thickness thereof to be created on thenanometer-scale range, especially for those situations in which anano-scale supporting wire 12 is employed, e.g., to promote greateremitter density. The CNT-polymer composite includes a polymer matrix anda plurality of carbon nanotubes uniformly dispersed therein. Usefully,the polymer is a material selected from a group consisting ofPolyethylene Terephthalate (PET), Polycarbonate (PC),Acrylonitrile-Butadiene Styrene Terpolyer (ABS), andPolycarbonate/Acrylonitrile-Butadiene Styrene Terpolyer (PC/ABS). Thepercent by mass of the carbon nanotubes in the CNT-polymer composite isin the approximate range from 0.05% to 10%. In one particularlyeffective embodiment, the percent by mass of the carbon nanotubes in theCNT-polymer composite is about 2%.

The CNT-glass composite includes a glass matrix and has a plurality ofcarbon nanotubes and conductive metal particles uniformly dispersedtherein. The conductive metal particles can, usefully, be silver orindium tin oxide (ITO). Quite advantageously, the conductive metalparticles are formed of silver or a silver alloy, and the mass of thesilver is about 15 times of that of the glass. If a silver alloy wereused, a high-purity (˜90 wt %+Ag) alloy would likely be most effective.In an effective embodiment, a length of the carbon nanotubes is in theapproximate range from 0.1 micrometer to 20 micrometers, a diameterthereof is in the approximate range from 0.5 nanometer to 100nanometers, and the percent by mass thereof in the CNT-glass compositeis in the approximate range from 0.2% to 10%.

Alternatively, the field emission layer 14 may, for example, be a CNTfilm manufactured by drawing out a bundle of carbon nanotubes, accordingto a certain width, from a super-aligned carbon nanotube array. Thebundles of the carbon nanotubes are, typically, connected together byVan Der Waals force interactions to form a continuous carbon nanotubefilm. In a useful embodiment, a diameter of the carbon nanotubes in thecarbon nanotube film is in the approximate range from 0.5 nanometer to100 nanometers, and a thickness of the carbon nanotube film is in theapproximate range from 5 nanometer to 10 micrometers. Quitebeneficially, the diameter thereof is in the approximate range from 5nanometers to 40 nanometers.

In use, a single field emission element 10 is fixed (e.g., via ametallurgical bond, such as a solder, or by a conductive adhesive) on/toa conductive cathode electrode (not shown), via the supporting wire 12thereof, to form a single field emission electron source. Furthermore, aplurality of such field emission elements 10 may be fixed on aconductive cathode electrode, via the respective supporting wires 12thereof, to form an array of field emission electron sources.Beneficially, the field emission layer 14 of the field emission element10 has an electrical connection with the conductive cathode electrode.With such a connection, voltage may be applied directly from theconductive cathode electrode to the field emission layer 14.Alternatively, voltage may be applied from the conductive cathodeelectrode to the field emission layer 14 via the supporting wire 12. Itis to be understood that the supporting wire 12 could enhance or,possibly, entirely provide the electrical connection of the fieldemission layer 14 with the conductive cathode electrode. At a minimum,at least one of the supporting wire 12 and the field emission layer 14must form an electrical connection with the cathode electrode to ensureoperability of the device.

Due to the carbon nanotubes in the field emission layer 14 having goodfield emission characteristics and the field emission layer 14 beingfixed on the conductive cathode electrode by the supporting wire 12, themechanical connection between the field emission layer 14 and thecathode electrode is firm, and the electrical connection therebetween issufficient. Thus, the electron emitting performance of the fieldemission element 10 is improved.

Referring to FIG. 3, a method for manufacturing the field emissionelement 10 includes the following steps:

(a): providing one supporting wire 12;

(b): forming at least one field emission layer 14 on an outer surface ofthe supporting wire 12; and

(c): cutting the supporting wire 12, with the at least one fieldemission layer 14 formed thereon, to a predetermined length and treatingthe supporting wire 12 to form the field emission element 10.

When a CNT-polymer composite is adopted as the field emission layer 14,the step (b) comprises the following steps:

(b1) adding and dispersing a plurality of carbon nanotubes in a meltedpolymer;

(b2) applying the nanotube-impregnated polymer on the outer surface ofthe supporting wire 12; and

(b3) cooling the nanotube-impregnated polymer to form the field emissionlayer 14.

The carbon nanotubes adopted in step (b1) can be obtained by aconventional method such as chemical vapor deposition, arc discharging,or laser ablation. Preferably, the carbon nanotubes are obtained bychemical vapor deposition. In step (b1), the carbon nanotubes areuniformly dispersed in the melted polymer by means of milling.

If a CNT-glass composite is adopted as the field emission layer 14, thestep (b) comprises the following steps:

(b1′) providing and mixing a plurality of carbon nanotubes, conductivemetal particles, organic carriers, and glass powder together to form acomposite paste;

(b2′) applying the composite paste on the outer surface of thesupporting wire; and

(b3′) drying and sintering the composite paste to form the fieldemission layer 14.

In step (b1′), the organic carriers is a mixture of terpineol and ethylcellulose. The mixture is formed at a temperature of about 80° C. bymeans of a water bath. In the mixture, the terpineol is acted as asolvent, and the ethyl cellulose acts as a stabilizing agent.Furthermore, the percent by mass of the terpineol in the mixture isabout 95%, and the percent by mass of the ethyl cellulose in the mixtureis about 5%. The carbon nanotubes adopted in step (b1′) can be obtainedby a conventional method such as chemical vapor deposition, arcdischarging, or laser ablation. Preferably, the carbon nanotubes areobtained by chemical vapor deposition.

Advantageously, the percent by mass of the organic carriers in the pasteis about 20%, the percent by mass of the conductive metal particles inthe paste is about 75%, and the percent by mass of the glass powder inthe paste is about 5%. The percent by mass of the carbon nanotubes inthe CNT-glass composite is in the approximate range from 0.2% to 10%. Inone particularly effective embodiment, the percent by mass of the carbonnanotubes in the CNT-glass composite is about 2%.

In step (b3′), the processes of drying and sintering is performed in theapproximate range from 300° C. to 600° C. The process of drying is usedto volatilize the organic carriers, and the process of sintering is usedto melt the glass powder to bond/adhere the conductive metal particleswith the carbon nanotubes.

Alternately, if a CNT film is adopted as the field emission layer 14,the step (b) comprises the following steps:

(b1″) selecting and drawing out a bundle of carbon nanotubes from a CNTarray to form the CNT film;

(b2″) wrapping the CNT film around the outer surface of the supportingwire 12; and

(b3″) immersing the supporting wire 12 in an organic solvent to form thefield emission layer 14.

Step (b1″) is executed as follows. Firstly, a super-aligned carbonnanotube array is provided. Secondly, a bundle of the carbon nanotubesaccording to a certain width is selected and drawn out from thesuper-aligned carbon nanotube array using forceps or anothergripping/pulling means, to form the carbon nanotube film along the drawndirection. The bundles of the carbon nanotubes are connected together byVan Der Waals force interactions to form a continuous carbon nanotubefilm.

It is to be noted that not all carbon nanotube arrays can be used tocreate the carbon nanotube films. The carbon nanotube films can only bedrawn out from the super-aligned carbon nanotube arrays. Based onextensive experimentation on the growth mechanisms of carbon nanotubes,the crucial factors for growing the super-aligned carbon nanotube arraysuitable for production of the films are listed below:

i) the substrate for growing the carbon nanotube array should besubstantially flat and smooth;

ii) the growth rate of the carbon nanotube array should be relativelyhigh; and

iii) the partial pressure of the carbon containing gas should berelatively low.

In general, a width and thickness of the carbon nanotube film can becontrolled by a size of a tip of the tool that is used to pull out thefilm. The smaller the tip is, the smaller the film width and thicknessis. A length of the carbon nanotube film depends on an area of thesuper-aligned carbon nanotube array. A force used to pull out the filmdepends on the width and thickness of the carbon nanotube film. Thebigger the width and thickness of the carbon nanotube film is, thebigger the force is. Preferably, the thickness of the carbon nanotubefilm is in the approximate range from about 5 nanometers to 10micrometers.

In step (b2″), a single layer or multiple layers of the CNT film can bewrapped around the outer surface of the supporting wire 12 to enclosethe outer surface of the entire supporting wire 12. In step (b3″), theprocess of immersing the supporting wire 12 in the organic solvent canensure the CNT film attaches firmly on/to the outer surface of thesupporting wire 12. Advantageously, the organic solvent is ethanol.

The field emission element 10 can be made directly according to theactual length needed. Alternatively, the field emission element 10 canbe made relatively long and then be cut according to the actual lengthneeded by means of, e.g., mechanical cutting or laser cutting.Furthermore, a surface treating process can be executed to the fieldemission element 10. The surface treating process can, for example, be alaser irradiating process and/or a mechanical rubbing process. Thissurface treating process can ensure at least part of or even all of thecarbon nanotubes dispersed in the polymer or glass open to the ambientat and/or proximate at least one end thereof. This exposure to ambientcan enhance the field emission performance of the carbon nanotubes.Furthermore, a large-current field emission aging process can beexecuted to the field emission element 10 to further enhance the fieldemission performance of the carbon nanotubes.

Compared with the conventional field emission element, the fieldemission element 10 of the present embodiment has the following virtues.Firstly, a field emission layer 14 of the field emission element 10adopts carbon nanotubes, and the carbon nanotubes have excellent fieldemission performance inherently. Thus, the field emission element 10 hasa relatively excellent field emission performance Secondly, thesupporting wire 12 can support and secure the field emission layer 14.This support ensures that the field emission element 10 has excellentmechanical characteristics. Thus, the field emission element 10 can bemade easily and used conveniently in field emission devices.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the invention. Variations maybe made to the embodiments without departing from the spirit of theinvention as claimed. The above-described embodiments illustrate thescope of the invention but do not restrict the scope of the invention.

1. A method for manufacturing a field emission element, the methodcomprising: (a) providing one supporting member; and (b) wrapping acarbon nanotube (CNT) film around an outer surface of the supportingmember at least once, the CNT film comprising a plurality of bundles ofcarbon nanotubes connected in series.
 2. The method as claimed in claim1, further comprising a step (c) of cutting the supporting member to apredetermined length after the step (b).
 3. The method as claimed inclaim 1, further comprising immersing the supporting member with the CNTfilm thereon in an organic solvent to firmly attach the CNT film on thesupporting member after the step (b).
 4. The method as claimed in claim3, wherein the organic solvent is ethanol.
 5. The method as claimed inclaim 3, further comprising treating an outer surface of the CNT filmafter immersing the supporting member with the CNT film thereon in theorganic solvent.
 6. The method as claimed in claim 5, wherein the outersurface of the CNT film is treated with a laser irradiating processand/or a mechanical rubbing process.
 7. The method as claimed in claim5, further comprising executing an aging process to the supportingmember with the CNT film formed thereon after treating the outer surfaceof the CNT film.
 8. The method as claimed in claim 1, wherein the CNTfilm is formed of pure carbon nanotubes.
 9. The method as claimed inclaim 8, wherein the supporting member is made of glass or ceramic. 10.The method as claimed in claim 1, wherein the supporting member iswire-shaped.
 11. The method as claimed in claim 1, wherein the CNT filmis formed by drawing a group of carbon nanotubes from a carbon nanotubearray.
 12. The method as claimed in claim 11, wherein the carbonnanotube array is a super-aligned carbon nanotube array.
 13. A methodfor manufacturing a field emission element, the method comprising thesteps of: (a) providing one supporting wire; (b) forming at least onefield emission layer on an outer surface of the supporting wire; and (c)cutting the supporting wire, upon forming the at least one fieldemission layer thereon, to a predetermined length to form the fieldemission element.
 14. The method as claimed in claim 13, wherein in step(c), the cutting process comprises a mechanical cutting process or alaser cutting process.
 15. The method as claimed in claim 13, wherein,after at least one field emission layer is formed on the supportingwire, a treating process is performed on the at least one fieldsubmission layer, the treating process comprising at least one of alaser irradiating process, a mechanical rubbing process, and alarge-current field emission aging process.
 16. The method as claimed inclaim 13, wherein the field emission layer is comprised of a materialselected from a group consisting of CNT-polymer composites, CNT-glasscomposites, and CNT films.
 17. The method as claimed in claim 16,wherein when a CNT film is adopted as the field emission layer, the step(b) comprises the following steps: (b1) selecting and drawing out abundle of carbon nanotubes from a CNT array to form the CNT film; (b2)wrapping the CNT film around the outer surface of the supporting wire toform at least one CNT film layer on the supporting wire; and (b3)immersing the supporting wire, with the at least one CNT film layerthereon, in an organic solvent to form the field emission layer.
 18. Themethod as claimed in claim 17, wherein either a single layer or multiplelayers of the CNT film are wrapped around the outer surface of thesupporting wire.
 19. The method as claimed in claim 16, wherein when aCNT-polymer composite is adopted as the field emission layer, the step(b) comprises the following steps: (b1′) adding and dispersing aplurality of carbon nanotubes in a melted polymer; (b2′) applying thepolymer, with the carbon nanotubes dispersed therein, on the outersurface of the supporting wire; and (b3′) cooling the applied polymer toform the field emission layer.
 20. The method as claimed in claim 16,wherein when a CNT-glass composite is adopted as the field emissionlayer, the step (b) comprises the following steps: (b1″) providing andmixing a plurality of carbon nanotubes, conductive metal particles,organic carriers, and glass powder to form a paste; (b2″) applying thepaste on the outer surface of the supporting wire; and (b3″) at leastdrying and sintering the paste to form the field emission layer.